tree 775b636093a744285f6226337a16d99020d1ee6d
parent d3f422c8d58b2f0d68a51db88389a715dc10b4d2
author Konstantin Khlebnikov <khlebnikov@yandex-team.ru> 1470335765 +0300
committer Jens Axboe <axboe@fb.com> 1470794286 -0600

mm, writeback: flush plugged IO in wakeup_flusher_threads()

I've found funny live-lock between raid10 barriers during resync and
memory controller hard limits. Inside mpage_readpages() task holds on to
its plug bio which blocks the barrier in raid10. Its memory cgroup have
no free memory thus the task goes into reclaimer but all reclaimable
pages are dirty and cannot be written because raid10 is rebuilding and
stuck on the barrier.

Common flush of such IO in schedule() never happens, because the caller
doesn't go to sleep.

Lock is 'live' because changing memory limit or killing tasks which
holds that stuck bio unblock whole progress.

That was what happened in 3.18.x but I see no difference in upstream
logic.  Theoretically this might happen even without memory cgroup.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Jens Axboe <axboe@fb.com>